[Rd] removeSource() vs. function literals

[Duncan Murdoch]

On 30/03/2023 10:32 a.m., Ivan Krylov wrote:
> Dear R-devel,
> 
> In a package of mine, I use removeSource on expression objects in order
> to make expressions that are semantically the same serialize to the
> same byte sequences:
> https://github.com/cran/depcache/blob/854d68a/R/fixup.R#L8-L34
> 
> Today I learned that expressions containing function definitions also
> contain the source references for the functions, not as an attribute,
> but as a separate argument to the `function` call:
> 
> str(quote(function() NULL)[[4]])
> # 'srcref' int [1:8] 1 11 1 25 11 25 1 1
> # - attr(*, "srcfile")=Classes 'srcfilecopy', 'srcfile'
> #   <environment:0x55aba55a8a50>
> 
> This means that removeSource() on an expression that would define a
> function when evaluated doesn't actually remove the source reference
> from the object.
> 
> Do you think it would be appropriate to teach removeSource() to remove
> such source references? What could be a good way to implement that?
> if (is.call(fn) && identical(fn[[1]], 'function')) fn[[4]] <- NULL
> sounds too arbitrary. if (inherits(fn, 'srcref')) return(NULL) sounds
> too broad.
> 

I don't think there's a simple way to do that.  Functions can define 
functions within themselves.  If you're talking about code that was 
constructed by messing with language objects, it could contain both 
function objects and calls to `function` to construct them.  You'd need 
to recurse through all expressions in the object.  Some of those 
expressions might be environments, so your changes could leak out of the 
function you're working on.

Things are simpler if you know the expression is the unmodified result 
of parsing source code, but if you know that, wouldn't you usually be 
able to control things by setting keep.source = FALSE?

Maybe a workable solution is something like parse(deparse(expr, control 
= "exact"), keep.source = FALSE).  Wouldn't work on environments or 
various exotic types, but would probably warn you if it wasn't working.

Duncan Murdoch